1. Field of the Invention
The invention relates to current controlled rate shaping for a unit pump fuel injector system (UPS) or a unit fuel injector system (UIS) for an internal combustion engine.
2. Background Art
Unit pump or unit injector fuel systems for internal combustion engines, particularly diesel engines, are designed to control fuel injection quantity and injection timing. By choosing an engine valve cam profile and a nozzle design, the parameters of a typical triangular fuel injection pressure time trace can be influenced to match the needs of the combustion process of a specific engine design. As national and international emission standards become increasingly stringent, deviations from an ideal fuel injecting process become less tolerable.
It has been demonstrated that it is advantageous with some engine designs to avoid raising the injection pressure for a period following initiation of the injection event, or to raise it at a lower rate, before resorting again during the injection event to a typical triangular fuel injection pressure time trace shape with a higher rate of increase of injection pressure. Fuel injection rate shaping in this fashion is an effective technique for reducing levels of particulates and oxides of nitrogen in the engine exhaust and for reducing engine noise.
In order to achieve added injection rate shaping functions, an additional stop for the control valve between a fully open position and a fully closed position of the control valve has been used, as disclosed in U.S. Pat. No. 6,276,610, to allow for a controlled leakage of pressurized fuel through the control valve.
Unit pump and unit injector high pressure fuel injection systems of known construction comprise a pump body with a pump plunger driven by a valve camshaft for an internal combustion engine. The pump body and the plunger define a high pressure fuel pumping chamber that is in fluid communication with injection nozzles. A control valve, which is situated in a hydraulic circuit between the high pressure pumping chamber and the nozzle, is stroked by an electromagnetic actuator between an open position and a closed position. The stroke range includes a fully open position, a fully closed position and a rate shape position close to the fully closed position of the control valve.
The control valve position during a fuel injection event is controlled by an electronic engine control module and an electromagnetic actuator for stroking the control valve. The module responds to engine operating variables. The module includes an electronic processor with software that defines a calibrated time period for the injection event and the time period between injection events. The time period for a given injection event is characterized by an intermediate or modified pressure in a time plot of the fuel pressure at the nozzle. The intermediate pressure precedes a main injection pressure pulse.
The control valve normally is stroked to a fully open position by a valve spring. When the actuator is energized, the control valve strokes towards a closed position against the opposing force of the control valve spring. A control valve stop comprising a spring loaded piston engages an adjacent end of the control valve. The stop also comprises a secondary stop shoulder that is in engagement with the valve when the valve is fully open. A spring in the control valve stop reduces the net force of the control valve spring. The spring rate of the control valve stop spring is less than the spring rate of the control valve spring. When the piston in the control valve stop hits a piston stop shoulder, typically after 120 xcexcm of stroke, a higher force is needed to pull the control valve to its fully closed position because the control valve stop spring does not oppose the control valve spring. That is due to a lack of contact between the control valve and the control valve stop piston during the final control valve travel of 30 xcexcm.
The actuator has an armature connected to the control valve. The control valve is urged towards the closed position against the force of the control valve spring when the actuator stator is energized.
In a typical embodiment, the stroke of the valve between a so-called rate shape position and the closed position may be about 30 microns. The full stroke of the control valve between the fully opened position and the closed position may be about 150 microns. Active rate shaping occurs when the valve is in contact with the stop piston, thereby causing a net reduction of the spring force acting on the valve body. The pressure gradient at that time is reduced because of the controlled leakage of fluid past the control valve seat.
It is possible that the control valve and the control valve stop, during assembly of the injector, may have their centerlines misaligned or skewed. This has a potential for changing the effect of the stroking of the stop piston because the valve may engage the secondary stop shoulder before the stop piston is fully displaced against the force of the stop piston spring. Any deviations in the alignment of the valve with respect to the stop piston can have an undesirable influence on injection rate shaping during the injection event with a resulting deterioration of engine emission quality.
The assembly of the control valve and the control valve stop of the invention is designed to compensate for any misalignment or lack of concentricity of the control valve with respect to the stop piston. The stop piston is in engagement with the control valve regardless of any misalignment or lack of concentricity due to manufacturing tolerances. Because such tolerances will not affect the rate shaping characteristics of the injection event, it is possible to assemble the control valve stop in a pre-assembly procedure during manufacture of the unit pump or unit injector. In a high-volume manufacturing operation, provision is made, furthermore, to reduce the number of potential fluid leakage paths through which fluid could leak into engine oil due to a damaged seal ring, for example.
According to a further feature of the invention, the need for precisely calibrating the control valve and control valve stop assembly is eliminated by providing calibrated shims for establishing controlled strokes of the stop piston and the control valve. Assembly of the shims can take place in a pre-assembly procedure for the control valve stop.
The unit pump or unit injector of the invention includes a pump body with a cylindrical pumping chamber defined in part by an engine camshaft-driven plunger. A fuel inlet passage and a fuel delivery passage communicate with a movable control valve.
The control valve has a land that engages a valve seat in the pump body. A spill flow path from the fuel delivery passage to a fuel return passage is established when the control valve is stroked toward its open position. A control valve spring strokes the control valve toward its open position and an electromagnetic actuator strokes the control valve toward its closed position.
The stop adjacent one end of the control valve defines a first stop position of the valve to limit total valve travel. A second stop position is defined by the spring-loaded stop piston, which is in engagement with the control valve when the control valve is stroked to an intermediate position as the rate of pressure increase in the fuel delivery passage is reduced. Misalignment of the control valve stop piston relative to the control valve is accommodated when the control valve engages the stop piston.